1. Field of the Invention
The present invention relates to a static mixer for placement in a flow channel and, more particularly, to an expanded grid static mixer for improved mixing of exhaust gases and a reducing agent in an exhaust emission control system of an internal combustion engine.
2. Description of Related Art
The use of a regulated or controlled diesel catalytic converter has proven to be an advantageous technique to reduce pollutants, particularly nitrogen oxides, in the exhaust of an internal combustion engine. This technique is used in particular in internal combustion engines operated with excess air, such as diesel and lean-mix engines. This technique, based essentially on the method of selective catalytic reduction (SCR), brings the nitrogen oxides into contact with a reducing agent on a selective catalyst, which converts them into harmless nitrogen and water.
An exhaust emission control system operating on this principle is discussed in a Siemens Aktiengesellschaft pamphlet entitled xe2x80x9cSiNOx Nitrogen Oxide Reduction for Stationary Diesel Engines,xe2x80x9d Order No. A96001-U91-A232, Siemens AG, Bereich Energieerzeugung (KWU), FreyeslebenstraBe 1, 91058 Erlangen, Germany (1994). Such a system can drastically reduce the nitrogen oxide emissions from a diesel engine. It mixes the nitrogen oxides in the exhaust gas with a reducing agent, usually ammonia obtained from urea by hydrolysis, in the exhaust pipe, and then introduces the mixture into a reactor space with a catalyst device for catalytic conversion. A static mixer, such as that shown in DE 41 23 161 (U.S. Pat. No. 5,489,153), is used for mixing the reducing agent with the exhaust gas.
Known static mixers have proven generally satisfactory in homogeneously mixing reducing agents with the exhaust gas in such exhaust emission control systems. However, the exhaust pipe configuration in a vehicle must be adapted to the vehicle""s structural design, and in many applications the minimum mixing path required by known mixers cannot be realized because of lack of space. In such a case, the flow conditions into the catalytic converter may be unfavorable, resulting in an uneven flow profile at the converter inlet.
It is an object of the present invention to avoid the shortcomings of prior art mixers in such situations.
The present invention relates to a static mixer for placement in the flow channel of an exhaust emission control system, which seeks to provide as short a mixing path as possible and align the flow direction of the exhaust gas in the flow channel.
In furtherance of the objects of the present invention, one aspect of the invention involves an exhaust system for an internal combustion engine having a catalytic converter for reducing pollutants in the exhaust gases from the engine in the presence of a reducing agent, which exhaust system comprises an exhaust duct for carrying the exhaust gases from the engine to the catalytic converter, the exhaust duct having an injection location for the introduction of the reducing agent into the exhaust gases, and a static mixer disposed in the exhaust duct upstream of the catalytic converter and proximate to the injection location, the static mixer comprising an expanded grid in the form of a mesh having a plurality of openings formed between crossbars of said expanded grid.
In accordance with another aspect of the present invention, an excess-air-operated internal combustion engine comprises a catalytic converter for reducing pollutants in the exhaust gases from the engine in the presence of a reducing agent, an exhaust duct for carrying the exhaust gases from the engine to the catalytic converter, an injection device in the exhaust duct for introducing the reducing agent into the exhaust gases upstream of the catalytic converter, and a static mixer disposed in the exhaust duct upstream of the injection device, the static mixer comprising an expanded grid in the form of a mesh having a plurality of openings formed between crossbars of the expanded grid.
Another aspect of the present invention is a method of mixing exhaust gases of an internal combustion engine with a reducing agent, the method comprising providing a static mixer comprising a conventional expanded grid in the form of a mesh having a plurality of openings formed between crossbars of the expanded grid, and placing the static mixer in an exhaust duct of the engine proximate to an injection location for the introduction of the reducing agent into the exhaust gases.